1. Field of the Invention
The present invention relates to the tempering of flat glass sheets and particularly glass sheets that are produced on a gas hearth of the type depicted in U.S. Pat. No. 3,332,759 to McMaster et al or U.S. Pat. No. 3,223,501 to Fredley et al. However, the present invention is also suitable for use in tempering flat glass sheets using a roller type conveyor of the type depicted in U.S. Pat. No. 3,923,488 to Pytel et al.
The present invention particularly relates to the tempering of flat glass sheets that have at least one dimension exceeding about 3.3 feet (1 meter). Glass sheets that are supported horizontally or substantially horizontally tend to develop a kink or curl at their leading edge. This upward curl is noticeable even when associated with relatively small glass sheets. However, when tempering glass sheets having one or more dimensions greater than about 3.3 feet (1 meter), the leading edge of the glass sheet develops an enlarged, upward curl. In such larger glass sheets and in sheets of smaller dimension heated on a gas hearth, the leading edge curl is usually sufficiently severe to cause the glass sheet to depart from the flat shape desired by an amount sufficient for the glass to be outside tolerance requirements. Such severe edge curl leads to customer rejects. Leading edge curl is the deviation from flatness of the glass sheet measured in the direction of its thickness in its initial one foot (30 centimeters) of length.
In well-known commercial procedures for tempering glass sheets, the sheets are first heated to a temperature corresponding substantially to the softening point of the glass and then are chilled rapidly to a temperature below the annealing range of the glass to impart a temper. Coventionally, the sheet is heated while transported through an enclosed hot atmosphere of a furnace and propelled forwardly through the furnace exit into a cooling station comprising upper and lower plenums which flush the opposite surfaces of the sheet with streams of suitable tempering medium, such as air under predetermined pressure applied through arrays of nozzles which chill the glass and impart the desired stress patterns therein. Cooling air is directed against the opposite surfaces of the sheet while the sheet moves between the upper and lower plenums in order to impart the temper desired.
It is also well known to differentially cool the upper and lower surfaces to either induce warpage or reduce the amount of warpage that results from the thermal treatment of the moving glass sheets. Warpage has been controlled both transverse to the direction of glass sheet travel and parallel to the direction of glass sheet travel by differential heating and/or cooling of the opposite surfaces.
2. Description of the Prior Art
U.S. Pat. No. 3,245,772 to Cypher et al discloses warping of a glass sheet by differentially heating its opposite major surfaces followed by cooling differentially to reduce the curvature or warpage induced by differential heating.
U.S. Pat. No. 3,396,000 to Carson et al discloses the use of blowers in the region intermediate a furnace exit and an entrance to a cooling station to induce a transverse curvature about an axis of glass sheet movement by applying blasts of cooling air downward through a plurality of obliquely disposed air delivery pipes. The pipes face the side edge portions only of glass sheets passing therebeneath.
U.S. Pat. No. 3,497,340 to Dennison et al discloses a technique for controlling the onset of longitudinal warpage to facilitate the passage of a glass sheet to be bent to a compound shape between upper and lower arrays of nozzles before the compound shape takes place so as to enable the glass sheet to be suitably cooled between upper and lower blasts of cooling medium applied through said nozzles.
U.S. Pat. No. 3,522,029 to Carson et al discloses the shaping of a glass sheet to a preselected curvature by differentially chilling one of its surfaces from the central area toward an edge area.
U.S. Pat. No. 3,923,488 to Pytel et al discloses a technique for reducing upward curl at the leading edge of a glass sheet conveyed through a heated area and a cooling area by applying a cooling medium toward a portion of the lower surface of the glass sheet only in spaced relation to its leading edge. This application is started after the glass sheet leaves the furnace and before the glass portion reaches a position where both its upper and lower surfaces are cooled by exposure to opposing blasts of tempering medium. The entire length of glass sheet except for the leading one foot or so of length is subjected to this differential cooling treatment.